Two solutions are prepared as follows:

A.- Citric Acid,...........................................................................

10 grammes.

Distilled Water,...........................................................................

100 "

B.- Caustic Soda,...........................................................................

10 grammes.

Distilled Water............................................................................

100 "

For each litre of silver solution to be corrected, two cubic centimetres of solution A are added, and the bath well agitated to disperse the citric acid through the liquid. Five cubic centimetres of solution B are next put in, the liquid being again shaken, and there becomes formed a brown precipitate of oxide of silver, which partially, or even entirely, disappears if the bath happened to be in an acid condition prior to the addition of the citric acid. A further quantity of five cubic centimetres of solution B is added, and if, after energetic agitation, no more of the brown precipitate disappears, no further quantity of the solution should be added.

The bath is now poured into a glass bulb without being filtered, and heated under a spirit-lamp until it boils. The solution will by this means become perfectly black by the precipitation of the organic matter, which, as we know, was the cause of the plates becoming fogged. The boiling should not, however, last for more than a minute, after which the solution is allowed to cool. - Wm. England.

167. Never work a bath after it has become necessary to add alcohol to the developer to avoid " crawling," but dump it into an evaporating-dish and boil the alcohol out of it. Let it get cold, and reduce to forty grains strength; add enough new solution to renew the original quantity, and the bath will not be over-iodized, and will work splendidly again without any sunning, unless it was also overcharged with organic matter. In that case the plates developed very weak, and there was difficulty in getting the developer to stay on the plate. The developer was

168. A method of testing the strength of a bath has already been given.

This for the photographer who really has some conscience and | little feeling for his stock - dealer.. For those who have not, the following note is given to show about how much can be honestly expected from a given nitrate boiling should have been continued until only a few ounces of solution remained, and sufficient oxide of silver (AgO) added to take up all the free acid, and proceed as before to reduce forty grains, and increase in quantity; then filter and sun until perfectly clear; filter again, and add fresh nitric acid as to a new bath. - F. M. Spencer.

The cold affects the chemicals by rendering them torpid, whereby they lose one - half of their power and energy. The collodion sets slowly, and the resulting plate, instead of coming out of the bath rich and creamy, will be thin and transparent, of a bluish color; the developer works slowly, depositing nearly all the silver upon the whites, thereby giving too great a density to the negative, and with difficulty bringing out the details. The result is an imperfect negative, with very little chance of doing better next time. Heat increases chemical action and cold decreases it. Cold renders long exposures necessary, and produces hard negatives without detail. On the other hand, too high a temperature will give flat negatives without contrast, with a tendency to spontaneous reduction, otherwise called fog. We must, then, to be successful, steer clear of both extremes. If possible, the temperature of the rooms should never be below 65° nor above 70° Fahrenheit. This temperature should be maintained as near as possible, night and day, by the aid of artificial heat More particularly is it necessary at night, because, when the heat is allowed to go down after the work of the day is done, the chemicals are all chilled by morning, and, although it may take but a few minutes for the room to get comfortable, it will take many hours for the heat to penetrate through the bath and solution, and just as they begin to get in tolerable working order it is time to close business, and you go again in the morning to find the same trouble.

Another strong reason for an even temperature is to be found in the fact, that a sudden rise of temperature in a cold room condenses moisture upon your negative glass and the lenses of your instrument, upon the same principle that it does upon a pitcher of ice-water when carried into a warm room. This moisture will cause the film to slip off your plates during the manipulations, unless they are warmed sufficiently to drive it off before coating. - George. H. FennEmorE.

Years ago I used to carry ice for miles, and have my bath stand in a box of ice, and imagined it was a great thing; but in many places it could not be got, no I had to look for other means of working in extremely hot days. First of all, I learned to keep my best bath for hot days - I always carry rwo - and, by judicious use of acid in my developer (putting it in in addition to what is already in the developer, after putting the developer in my deting-cup, altering the charge of acid as I see I need from one exposure to another get along. Next be careful and not over - expose, for this is one great source of failure. In hot weather a plate but slightly over - exposed has lost its snap, and cannot be relied on for a brilliant negative. - B. W. Kilburn.

168. A 12 x 10 glass bath, holding about 80 ounces of new solution 85 grains to the ounce, and made with distilled water and three drops of nitric acid, did 218 plates 71/2 x 71/2 inches, log about 60 square feet It gave up suddenly, needle-like crystals being formed all over the plate, on the inside of the bath, and on the dipper. Before this point was reached, the of silver solution, and how much more may not be expected from it. And with it well digested you are in condition to investigate further troubles. 169. Collodion is liable to frequent and rapid changes, and needs to be continually modified to suit the condition of the bath. This is easily done, however. The best remedy for a pale, neutral collodion is to take from the stock-bottle as much as will last for the day, and add to it a little old red collodion until it assumes a rich orange color; or a drop or two of tincture of iodine will do as well. Let it stand a few minutes, when a great improvement will be found. On the other hand, if the positive picture is clear and brilliant, and the negatives are foggy that are made by day, the trouble undoubtedly comes from light penetrating in some place where it ought to be excluded; the only remedy for which is to find the enemy, put him out, and keep him out.